Final answer:
Cyclin-dependent kinases (Cdks), in partnership with cyclins, are the internal molecular signals that determine if the cell cycle should proceed at various checkpoints.
Step-by-step explanation:
The internal cues that decide whether the cell cycle should proceed are cyclins and cyclin-dependent kinases (Cdks). These molecules act as molecular signals that regulate the transitions of the cell through the different checkpoints in the cell cycle, such as the G1 checkpoint for DNA replication readiness, the G2 checkpoint for mitosis preparation, and the metaphase checkpoint during mitosis.
Specifically, Cdks must bind to a cyclin and be phosphorylated by another kinase to become fully activated and push the cell cycle forward, while tumor suppressor genes act as negative regulators that can halt the cell cycle under certain conditions. Therefore, the correct answer is C) Cyclin-dependent kinases (Cdks).